1. Field of the Invention
The present invention relates to a Semiconductor Bulk Acoustic Resonator (SBAR) or Bulk Acoustic Wave (BAW) resonator configured to reduce lateral acoustic modes in order to improve the insertion loss and phase characteristics of SBAR filters.
2. Description of the Prior Art
Semiconductor bulk acoustic resonators (SBARs) are known in the art. An example of such an SBAR is disclosed in the commonly owned U.S. Pat. No. 5,382,930; incorporated by reference. SBAR devices include resonators, stacked crystal filters (SCFs), multipole filters based on inductor coupled topologies disclosed in the ""930 patent and other filter topologies. Because of their relatively small size, high Q, and high operating frequency, such SBARs are particularly well suited in high performance fitting applications where space is limited.
Such SBARs are thin film acoustic resonators fabricated on a semiconductor substrate, such as gallium arsenide (GaAs), making them particularly suitable for integration with high electron mobility transistor (HEMT) and heterojunction bipolar transistor (HBT) microwave monolithic integrated circuits (MMICs), for example, as disclosed in: xe2x80x9cSBAR Filter Monolithically Integrated With an HBT Amplifierxe2x80x9d, Cushman et al., IEEE Ultrasonic Symposium, 1990, pp. 519-524; and xe2x80x9cSBAR-HEMT Monolithic Receiver Front End With Bulk Acoustic Filtersxe2x80x9d, Cushman et al., GOMAC Digest, 1997, pp. 279-282; hereby incorporated by reference. Such SBARs typically include a thin layer or film of a piezoelectric material, such as aluminum nitride or zinc oxide which may be deposited on the semiconductor substrate, for example, by sputtering. Thin film metal electrodes are formed on opposite surfaces of the piezoelectric layer to form an SBAR resonator. A stacked crystal filter (SCF) is similar but includes two AIN layers and three metal electrodes.
Irregularities of insertion loss and phase have heretofore prevented such SBARs from being used in many applications. Such performance irregularities are due to the undesired lateral acoustic modes of such SBARs. These lateral acoustic modes are known to interfere with the desired longitudinal acoustic mode thereby causing the performance irregularities discussed above.
It is an object of the present invention to solve various problems in the prior art.
It is yet another object of the present invention to provide an (SBAR) filter with improved performance characteristics.
It is yet a further object of the present invention to provide an SBAR that is configured to suppress lateral acoustic modes.
Briefly, the present invention relates to a semiconductor bulk acoustic resonator (SBAR) with improved passband insertion loss and phase performance. The SBAR is configured to suppress lateral propagating acoustical wave modes. The lateral acoustical wave modes are controlled by varying the lateral dimension of the resonator electrodes and or utilizing a visco-elastic acoustic damping material, such as polyimide, along at least a portion of the perimeter of the electrodes to attenuate reflections of the lateral acoustic modes at the electrode edges back into the electrode region.